1. Field of the Invention
The present invention relates to an inspection apparatus using an optical interferometer for inspecting information concerning distribution of optical constant of a sample such as refractive index, extinction coefficient or the like as well as geometrical size of a sample or the like by laying a sample of a biomedical tissue or the like under exposure of light and detecting light scattered or reflected by the sample by using an optical interferometer.
2. Description of the Related Art
As examples of report of apparatus with an object of measurement concerning optical property or geometrical size of a sample by laying a sample of a biomedical tissue or the like under exposure of low coherent light and detecting light scattered or reflected by the sample, there are SPIE, Vol. 1889, 1993, pp. 197-211 (Literature 1), OPTICS LETTERS, Vol. 21, 1996, pp. 1839-1841 (Literature 2) and Proceedings of No. 57 Applied Physics Society, No. 0 separate volume, p.1213 (Literature 3).
According to Literature 1, light scattered or reflected by a sample is detected by using an optical interferometer for splitting light from a super luminescent diode (SLD) and modulating respective split light by respectively different frequency by an acousto-optic modulator. The acousto-optic modulator can provide pure frequency shift to light, a processing of a signal detected by the optical interferometer is carried out by narrow-band reception by a single frequency or lock-in detection in synchronism with a beat signal and the signal processing is comparatively simple. However, there are caused drawbacks in which a constitution of an optical system is complicated and large-sized, alignment of the optical system is troublesome and measurement of a sample by multi-wavelength light is troublesome and so on.
According to Literature 2 and Literature 3, light scattered or reflected by a sample is detected by using an optical interferometer for modulating reference light by using PZT (piezoelectric vibrator) as a phase modulator. According to Literature 2, an output from the interferometer is made to pass through a band-pass filter, and subjected to envelope detection, successively, sampling by an A-D converter is executed. According to Literature 3, an output from the optical interferometer is made to pass a single mode optical fiber, interfered light is filtered, only reflected straight moving light is detected by a photodiode, thereafter, the light is made to pass a high-pass filter and sampling by an A-D converter is executed. According to modulation of reference light by using PZT, a constitution of an apparatus is comparatively small-sized, alignment of an optical system is facilitated and expansion to measurement of a sample by multi-wavelength light is not so difficult.
According to modulation of reference light using PZT as a phase modulator in the above-described conventional technology, optical-paths of side band wave and carrier wave are the same and the side band wave and the carrier wave cannot be separated from each other and accordingly, there poses a problem in which pure frequency shift cannot be provided continuously to the reference light, some phase modulation is provided to the reference light and a stable signal cannot be detected by only receiving by the narrow-band reception in a single frequency, an output signal from a photo detector detected by the optical interferometer.
The conventional technology using envelope detection and a high-pass filter cannot be regarded as a technology sufficient for achieving detection of a stable signal, compared with narrow-band reception in a single frequency or lock-in detection in synchronism with a modulated signal, reception band of a signal is far wide (for example, a receiving band width of a signal is wide by 1000 times or more in comparison with lock-in detection with time constant of 1 sec when frequency is 1 KHz) and therefore, one cannot make full use of the characteristic for executing detection of light with high-sensitivity by using the optical interferometer (in principle, detection of single photon is feasible) and there cannot be achieved a signal-to-noise ratio near to its theoretical limit (when notation h designates Plank""s constant, notation B designates band width of detecting system, notation P designates power of signal light and notation xcexd designates frequency of light, signal-to-noise ratio=S/N=P/(hxcexdB) and the S/N may be referred to as quantum noise limit).
It is an object of the present invention to provide an inspection apparatus using an optical interferometer for executing optical detection by an optical interferometer using a general optical modulator providing phase modulation such as PZT, an electro-optic modulator or the like to thereby simultaneously enable stable signal detection and narrow-band reception of signal in order to execute high-sensitivity detection of light using the optical interferometer by a signal-to-noise ratio near to its theoretical limit.
According to an inspection apparatus using an optical interferometer of the present invention, incident light is made to be irradiated on a predetermined point of a sample and light scattered or reflected by the sample is detected as signal light by an optical interferometer, when a fundamental frequency of phase modulation provided to 1 or 2 or 3 of the reference light, the incident light and the signal light is set to f, in an output signal from a photo detector, since one of first signal components having frequencies of multiples of odd numbers of the fundamental frequency f of the phase modulation and at least one of second signal components having frequencies of multiples of even numbers of the fundamental frequency f of the phase modulation are detected and relative intensity of the signal light is calculated from the amplitudes of the first and the second signal components.
According to a blood glucose level monitoring apparatus of the present invention for monitoring a blood glucose level in a biomedical tissue by detecting a glucose concentration in the biomedical tissue, there are provided an optical interferometer including a light source for emitting light having a predetermined wavelength absorbed by glucose in a visible, near infrared, infrared (500 nm through 2000 nm) range, splitting and combining means for splitting light from the light source into incident light irradiated on the biomedical tissue and reference light and combining signal light which is light scattered or reflected by the biomedical tissue and the reference light, a modulator for subjecting the signal light to phase modulation, and a photo detector for detecting light combined by the splitting and combining means, further including first detecting means for detecting an amplitude V1 of a first signal component having a frequency of a multiple of one of the fundamental modulation frequency of the modulator, second detecting means for separating and detecting an amplitude V2 of a second signal component having a frequency of a multiple of two of the fundamental modulation frequency of the modulator in a signal from the photo detector and means for calculating an intensity of the signal light V* by the following equation       V    *    =            1      2        ⁢                                        V            1            2                                              J              1              2                        ⁡                          (              θ              )                                      +                              V            2            2                                              J              2              2                        ⁡                          (              θ              )                                          
where J1 and J2 designate Bessel functions and xcex8 designates an amplitude of phase modulation of the signal light, by using the amplitudes of the first and the second signal components, in which the glucose concentration is detected by calculating an optical-path length between a plurality of interfaces of the tissue in the biomedical tissue and attenuation of light between the plurality of interfaces. Further, according to the blood glucose level monitoring apparatus of the present invention, the amplitude V1 of the first signal component and the amplitude V2 of the second signal component are calculated by subjecting the signal from the photo detector to Fourier transformation by a Fourier transform device.